1. Field of the Invention
Embodiments of the present invention relate generally to the field of optical coherence tomography and applications thereof. Specifically, embodiments of the present invention relate generally to methods and systems for measuring the geometric properties of the cornea.
2. Description of Related Art
The cornea and associated tear film are the primary refractive elements of the eye and the shape of the cornea is exceptionally important for vision. The shape of the cornea is commonly impacted in ectactic diseases, such as keratoconus, and in refractive and other surgical procedures.
Conventionally, the shape of the anterior surface of the cornea is measured using the principle of placido rings imaging. FIGS. 1A and 1B show an example of topographic imaging using the principle of placido rings imaging. As is commonly known in the arts, concentric rings 120 are projected onto the anterior corneal surface 110 of the eye 100, which is a convex and reflective surface. The variation in size of the virtual images of these reflected rings 120 can be used to derive the shape and refractive power of the anterior corneal surface, such as the axial map of the corneal curvature 150 derived using information from reflected ring positions as shown in FIG. 1B. Several corneal topography devices are commercially available using the placido rings imaging principle. In these devices, several topographic maps representing the derived anterior cornea surface are usually displayed, such as an axial/sagittal power or radius of curvature, tangential/instantaneous power or radius of curvature, and elevation relative to a reference surface. Although these topographic maps are commonly assumed to represent the shape of the anterior cornea surface, in actuality these topographic maps measure the shape of the corneal tear film, which is the first and primary reflective surface of the anterior surface of the eye. Therefore, topographic maps using the placido rings imaging principle can be disrupted in cases of poor or irregular tear film, especially those associated with dry eye conditions.
FIG. 2 is a diagram showing the anatomy of the cornea. The outer corneal epithelium 210 (or the anterior cornea), defining the outermost layer of the cornea 200, is a dynamic tissue which can remodel the cornea surface in the case of corneal ectactic disease or after refractive surgery. Changes in the thickness of the outer corneal epithelium 210 can mask changes in the underlying shape, such as the curvature of the corneal stroma 220 which is important in assessing corneal ectactic disease and corneal refractive surgical procedures. The inner corneal endothelium 230 defines the innermost layer of the cornea 200. In-between the outer corneal epithelium 210 and the corneal stroma 220 is the Bowman's membrane or the stromal-epithelial interface 215; while in-between the corneal stroma 220 and the inner corneal endothelium 230 is the Descemet's membrane 225. For example, epithelial thinning over an ectactic corneal stroma may prevent the detection of forme fruste keratoconus and other early ectactic disease important in the screening for refractive surgical procedures.
FIG. 3 is an exemplary schematic showing the effect of epithelial remodeling of the cornea. In the cornea 300 of FIG. 3, the corneal stroma 320 is deformed at deformation 350 due to some ectactic disease. However, the anterior corneal surface 340 is still relatively smooth and uniform due to the dynamic remodeling of the corneal epithelium 310. In these cases, measurement of the shape of the anterior corneal surface 340 alone, as performed by conventional placido topography, may not reveal the subtle changes in the shape of the corneal stroma, such as the damaged corneal stroma 320 in FIG. 3, as these changes may be masked by compensatory changes in the thickness of the corneal epithelium 310. On the other hand, an advantage of placido topography is its high sensitivity to small changes in corneal curvature as small changes in corneal height usually translates into larger measureable changes in the ring positions.
Epithelial remodeling may cause refractive regression after corneal laser refractive procedures. Also, refractive regression may be caused by changes in the shape of the cornea stroma which could indicate a structural weakness in the cornea. Measurements of the anterior corneal surface alone using conventional placido rings principles may not be able to distinguish these different causes of regression which are important in assessing corneal ectactic disease and corneal refractive surgical procedures.
One method of deriving additional information concerning the shape of the cornea stroma is to measure the shape of the posterior corneal surface, as the corneal endothelial thickness remains generally constant, contrary to the dynamic remodeling nature of the corneal epithelial layer as discussed above. Several commercially available clinical instruments attempted to measure the shape, such as curvature, of the posterior corneal surface. The Orbscan (Bausch & Lomb, Rochester, N.Y.) uses placido rings to measure the anterior corneal surface, and a scanning slit beam to determine conical thickness. Both measurements are used to derive the posterior corneal topography. The Pentacam (Oculus, Arlington, Wash.) employs the principle of Scheimpflug photography to measure both the anterior and posterior surfaces of the cornea. The Galilei (Zeimer, Alton, Ill.) uses a combination of placido rings imaging and Scheimpflug photography to generate topographic maps of both the anterior and posterior corneal surfaces. However, the spatial resolution of all these instruments is inadequate to accurately measure the shape and thickness of various tissue layers, such as the corneal epithelium, the corneal stroma, and the stromal-epithelial interface.
High-resolution cross-sectional imaging techniques, such as optical coherence tomography (OCT) and high-frequency ultrasound, have been used to measure the corneal epithelial thickness. Corneal epithelial thickness may be measured directly from OCT images using a computer algorithm available in commercial instrumentation, for example, in the RTVue (Optovue, Fremont, Calif.). Some methods were proposed to guide laser corneal surgery using OCT measurements of the corneal epithelial thickness. Some other methods disclose using either OCT, ultrasound, or Scheimpflug photography to map corneal epithelial thickness prior to laser epithelial ablation. Apparatus was also proposed to use high frequency ultrasound to measure corneal tissues thicknesses, including the epithelium and stroma. However, clinically useful measurement and data representation and display of the shape, such as curvature, of the corneal stromal/epithelial interface, using topographic maps of axial/sagittal power or radius of curvature, tangential/instantaneous power or radius of curvature, mean curvature, elevation, and elevation relative to a reference surface, in a manner similar to what a clinician is accustomed to in a routine clinical practice, are not available.
Therefore, methods and apparatus to obtain measurements of the corneal stroma, and in particular, to derive the shape of the anterior stromal/epithelial interface, and to display them using a topographic map in a similar manner to conventional mapping of the anterior corneal air/tear film interface are needed.